Method and Apparatus for Detecting Pressure Distribution in Fluids

ABSTRACT

A pressure sensing apparatus has a light source for transmitting pulses of light along a monomode optical fibre. The polarisation of light backscattered from the light pulses in the optical fibre is detected by a polarisation processing unit (PPU) and a photo detector. The optical fibre is adapted to deform asymmetrically under the influence of applied external isotropic pressure, e.g. from a fluid. The deformation causes the birefringence of the optical fibre to change proportionally to the applied pressure. The change in birefringence can be determined from the detected polarisation of the backscattered light, allowing detection of pressure distribution in the fluid. Importantly, the construction of the optical fibre is such that the birefringence beat length of the optical fibre at the wavelength of light propagated by the fibre remains more than twice the spatial length of the light pulses transmitted along the optical fibre. Applications of the invention include detection of fluid flow; location of an interface between two fluids of different density; and tsunami detection.

FIELD OF THE INVENTION

This invention relates to a method and apparatus for detecting pressuredistribution in fluids, particularly, but not exclusively, by analysinga single mode side-hole optical fibre using polarisation optical timedomain relfectometry. The invention also includes methods and apparatusfor measuring fluid flow; determining the location of an interfacebetween two fluids of different densities; and detecting tsunamis.

BACKGROUND TO THE INVENTION

When external pressure is applied to an optical fibre, the core of thefibre experiences strain which varies with the applied pressure. Due toa phenomenon known as the “elasto-optic effect”, the way in which lightpropagates in the fibre's core changes as the strain experienced by thecore changes. So, by looking at the way in which the light propagationproperties of the fibre's core change, changes in strain and hencepressure applied to the fibre can measured.

Most conventional optical fibres have circular symmetry in cross sectionand, indeed, are uniformly cylindrical. This means that when externalisotropic pressure is applied to them, e.g. when they are immersed influid, mechanical stress of substantially the same magnitude in everydirection orthogonal to the length of the fibre is applied to thefibre's core. Any deformation of the core is therefore substantiallyuniform orthogonal to its length. In addition, the silica from whichoptical fibres are usually made is relatively incompressible. The strainexperienced by the fibre's core is therefore relatively small for agiven pressure and the optical properties of standard optical fibresonly vary very slightly with changes in external isotropic pressure.This insensitivity makes them fairly impractical for pressuremeasurement.

It has therefore been suggested to use optical fibres adapted toexperience asymmetrical strain at their cores under the influence ofisotropic external pressure for measuring pressure in fluids. One suchfibre is known as a side-hole fibre. A side-hole fibre typically has twoair holes that extend parallel to the core along the length of thefibre, the holes being positioned on either side of the core. Thepresence of the air holes means that the core of the fibre experiencesless stress in a direction transverse to its length that extends betweenthe air holes than in other directions transverse to its length whenisotropic external pressure is applied to the fibre. Indeed, thedirection of greatest stress is that orthogonal to a line extendingbetween the air holes. The core therefore experiences greater strain inthis direction than others. One consequence of the asymmetrical strainexperienced by side-hole fibres is that the refractive index of thefibre's core changes more for light linearly polarised in the directionof greatest strain than for light linearly polarised in the orthogonaldirection. In other words, the fibre's birefringence changes, with theso-called fast axis in the direction of greatest strain. The change inbirefringence is proportional to the applied external isotropic pressureand large in comparison to changes in optical properties of standardoptical fibres. Side-hole fibres are therefore much more useful forpressure measurement than standard fibres.

Changes in fibre birefringence can be measured in a variety of ways. Onewell established method involves use of a Bragg grating written in thefibre, often referred to as a Fibre Bragg Grating (FBG). FBGs aredescribed, for example, in the paper “Fiber Bragg Grating TechnologyFundamentals and Overview”, Kenneth O. Hill et al, Journal of LightwaveTechnology, Vol. 15, No. 8, August 1997. Briefly, an FBG comprises aperiodic modulation in the isotropic refractive index of a fibre's corealong the length of the core. This modulation can be written into thecore using interfering coherent light, e.g. using an ultra violet (UV)laser and an appropriate optical arrangement to create a standing wavein the fibre that selectively heats the fibre's core to change itsrefractive index at the desired locations to create a grating. Whenlight travels along an FBG, some of it is reflected, with the reflectionoccurring most strongly at a wavelength of light known as the Braggwavelength λ_(B), which can be expressed asλ_(B)=2nΛ_(B)where n is the refractive index of the fibre and Λ_(B) is the period ofthe grating. Changes in refractive index of the fibre therefore causeproportional changes in the Bragg wavelength λ_(B). Indeed, a change inthe birefringence of a side-hole fibre at the FBG results in there beingeffectively two Bragg wavelengths, with one wavelength being moresensitive to changes in applied external isotropic pressure than theother. This allows measurement of applied external isotropic pressureand some compensation for other influences on fibre birefringence, suchas temperature. This is described in more detail, for example, in thepaper “Thermally Insensitive Pressure Measurements up to 300° C. UsingFiber Bragg Gratings Written on to Side Hole Single Mode Fiber”, TsutomuYamate et al, SPIE Proceedings, Vol. 4185, p 628, 2000.

Another method involves the use of a polarisation rocking filter. Thisis described, for example, in the paper “Approach to Highly SensitivePressure Measurements Using Side-Hole Fibre”, J. A. Croucher et al,Electronics Letters, Vol. 34, No. 2, 22 Jan. 1998. Briefly, unlike anFBG, which comprises a periodic modulation of a fibre core's isotropicrefractive index, a rocking filter comprises a periodic modulation ofthe core's birefringence. A rocking filter converts light travellingalong the filter between two orthogonal polarisation states. Thewavelength λ_(R) at which this conversion occurs most strongly, i.e. theresonant wavelength, can be expressed asλ_(R)=bΛ_(R)where b is the fibre birefringence and Λ_(B) is the pitch of thegrating. So, as the birefringence b of a side hole fibre in which arocking filter is written changes, the wavelength λ_(R) of light moststrongly converted from one polarisation state to the orthogonalpolarisation state as it passes through the rocking filter changesproportionally. Again, this allows measurement of external isotropicpressure applied to the fibre. It has been found that the use rockingfilters can allow measurements up to 80,000 times more sensitive tochanges in applied pressure than measurements using FBGs.

However, the use of side-hole fibres incorporating either FBGs orrocking filters has a number of problems. In particular, pressure canonly be measured at the location of the FBG or rocking filter. Thismeans that the measurement is restricted by the size and location of theFBG or rocking filter along the length of the fibre. This significantlylimits the usefulness of optical fibres for pressure measurement.

For example, an FBG typically has a pitch Λ_(B) below 1 μm and light isstrongly reflected by an FBG of only a few cm in length. So, changes inapplied external pressure measured using an FBG only relate to thepressure applied to the fibre over a few cm of its length. Furthermore,it is very difficult to measure the pressure applied to a fibre atmultiple positions along its length using FBGs, as the provision and useof multiple FBGs in a single optical fibre is limited and complex. Forexample, in order to distinguish between different FBGs, the differentFBGs need to be addressed using light of slightly different wavelengths,say around 2 nm apart from one another. As it is only possible tointerrogate the fibre with light having a limited wavelength range, sayaround 60 nm, the number of different FBGs that can be used in a singlefibre is severely limited, say to around 30. This can be mitigated tosome extent by looking at light reflected at different times from alight pulse travelling along the fibre to distinguish between lightreflected at different FBGs. In other words, some wavelength re-use canbe achieved using time division multiplexing. However, it is impossibleto avoid totally influence by other FBGs on the light reflected in oneFBG. In other words, cross talk between the FBGs inevitably occurs. Evenusing time division multiplexing, the maximum likely number of FBGs thatcould be used in a single fibre is therefore around 100.

Similar problems occur with rocking filters. The pitch λ_(R) of atypical rocking filter written in a side-hole fibre is longer (e.g. atleast 2000 times longer) than that of an FBG and can range from a fewmillimetres to a few metres. This means that measurements of appliedpressure using a single rocking filter generally relate to the averagepressure over a considerable length of the fibre. It is also difficultto measure the pressure applied to a fibre at multiple positions alongits length as the provision and use of multiple rocking filters in asingle fibre is limited and complex. Like FBGs, the number of rockingfilters that can be used in a single fibre is limited by wavelengthrestrictions and crosstalk problems, with again only a very few rockingfilters being useable in a single fibre.

Another problem is that the fabrication of side-hole fibresincorporating FBGs or rocking filters is relatively complex andexpensive. More specifically, writing FBGs and rocking filters inoptical fibres is a difficult and time consuming process. Sensor headsincorporating FBG and rocking filter fibres can therefore be relativelyexpensive. This is a particular problem when the sensor heads are usedin harsh or dangerous environments in which they are likely to bedamaged.

The present invention seeks to overcome these problems.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is providedan apparatus for detecting pressure distribution in a fluid, theapparatus comprising:

a single mode optical waveguide for immersion in the fluid, the opticalwaveguide being adapted to deform asymmetrically transverse to itslength on exertion of substantially isotropic pressure by the fluid suchthat birefringence of the waveguide varies with the exerted pressure;

a light source for directing pulses of polarised light into thewaveguide;

a polarisation detector for detecting polarisation states of lightbackscattered from the light pulses in the waveguide; and

a processor for deriving an indication of the birefringence of at leastpart of the waveguide from the detected polarisation states to detectpressure distribution in the fluid.

According to a second aspect of the present invention, there is provideda method of detecting pressure distribution in a fluid using a singlemode optical waveguide immersed in the fluid, the optical waveguidebeing adapted to deform asymmetrically transverse to its length onexertion of substantially isotropic pressure by the fluid such thatbirefringence of the waveguide varies with the exerted pressure, themethod comprising:

directing pulses of polarised light into the waveguide;

detecting polarisation states of light backscattered from the lightpulses in the waveguide; and

deriving an indication of the birefringence of at least part of thewaveguide from the detected polarisation states to detect pressuredistribution in the fluid.

So, the optical waveguide is adapted to experience greater strain in onedirection across its length (e.g. for an optical fibre, across its core)than in other directions across its length under influence of externalisotropic pressure. This means that the birefringence of the waveguidechanges in proportion to the pressure of the fluid in which it isimmersed. An example of a suitable waveguide is a side hole opticalfibre.

Changes in the birefringence of the waveguide can be detected by lookingat the polarisation states of light backscattered (e.g. by Rayleighbackscattering) in the waveguide. One technique for doing this is knownas Polarisation Optical Time Domain Reflectometry (POTDR), as described,for example, in International patent publication no. WO 02/095349 andthe paper “Measurement of the Spatial Distribution of Birefringence inOptical Fibers”, Wuilpart et al, IEEE Photonics Technology Letters, Vol.13, No. 8, August 2001.

Backscattering and, in particular, Rayleigh backscattering (which iscaused by small imperfections and inhomogeneities in the propagationmedium of the waveguide, e.g. the silica core of an optical fibre)occurs naturally in optical waveguides. So, it is possible to derive anindication of the birefringence of the waveguide at any point along itslength from the polarisation states of light backscattered in thewaveguide. Unlike pressure detectors using FBG and rocking filterfibres, the invention is not therefore limited to detecting pressure atpredetermined locations along the length of the waveguide, e.g. where anFBG or rocking filter is positioned. Rather, pressure can be detected atany selected location(s) along the length of the waveguide or over theentire length of the waveguide. This makes the invention far more usefulfor the detection of pressure differential.

So, the at least part of the waveguide for which an indication ofbirefringence can be derived can be the whole length of the waveguide;or (a) selected part(s) of the length of the waveguide. This is achievedsimply by detecting the polarisation states of light backscattered atappropriate positions along the length of the waveguide. It is thereforepreferred that the polarisation detector is able to detect thepolarisation state of light following backscattering at different pointsalong the length of the waveguide and that the at least part of thewaveguide for which the processor derives an indication of birefringenceis between at least some of these points.

It can also be appreciated that the invention can be used to detecteither or both temporal or spatial pressure distribution(s). Forexample, the processor can derive an indication of the variation ofbirefringence along the waveguide to detect spatial pressuredistribution in the fluid. Similarly, the processor can derive anindication of birefringence of the at least part of the waveguide morethan once to detect temporal pressure distribution in the fluid. Theterm “pressure distribution” is therefore intended to refer to both timevarying and spatially varying pressure in the absence of furtherlimitation.

Rayleigh backscattering tends to be a relatively strong phenomenon incomparison to other scattering phenomena, such as Raman and Brillouinscattering, which may be around 30 to 40 dB weaker and around 15 to 20dB weaker respectively. This means that the invention can takemeasurements from the backscattered light more quickly than apparatusdesigned to use Raman or Brillouin scattering, and results can beobtained faster.

Yet another advantage is that the waveguide of the invention, and hencea sensor head incorporating the waveguide, is relatively simple andcheap to manufacture, as the waveguide does not have a grating of anysort written in it.

The birefringence of the waveguide broadly depends on two factors: thewaveguide's intrinsic birefringence and its induced birefringence.Intrinsic birefringence is generally considered to be the birefringenceof the waveguide in the absence of any external influences on thewaveguide, such as externally applied stress and pressure, magnetic andelectric fields or temperature variation. For example, intrinsicbirefringence of the waveguide is typically determined at a neutralpressure (e.g. atmospheric pressure). For an optical fibre, intrinsicbirefringence results, e.g., from inhomogeneity in the materials fromwhich the fibre is made; variations in the fibre's geometry along itslength; and strain occurring in the fibre's core in the absence ofexternal influences. The waveguide's induced birefringence is a changein the waveguide's birefringence caused when external isotropic pressureis applied to the waveguide, e.g. when it is immersed in the fluid. Ofcourse, in a broader context, it is basically the component of thewaveguide's birefringence caused by external influences.

In order to maximize the sensitivity of FBGs and rocking filters tochanges in applied pressure, it is necessary to use optical fibres thathave high intrinsic birefringence or at least demonstrate very largechanges in induced birefringence with applied pressure. However, opticalwaveguides having high birefringence are not well suited to use with theinvention. The higher the birefringence of the waveguide, the shorterthe length of the light pulses that need to be used to successfullyresolve polarisation states useful in deriving waveguide birefringence.However, the light pulses cannot be too short, as this makes it hard todetect light backscattered in the waveguide. More specifically, shorterlight pulses must have greater intensity and this leads to unwanted andconfusing non-linear effects in the backscatter process. The opticalwaveguide of the invention preferably therefore has low birefringence.More specifically, it is preferred that the birefringence of the opticalwaveguide remains low over the range of fluid pressures in which it isintended to use the waveguide. This is considered to be new in itselfand, according to a third aspect of the present invention, there isprovided an optical waveguide for use in detecting pressure differentialin a fluid, the waveguide being adapted to deform asymmetricallytransverse to its length on exertion of substantially isotropic pressureby the fluid such that the birefringence of the waveguide varies withthe exerted pressure, but remains low over the range of fluid pressuresin which it is intended to use the waveguide. Likewise, according to afourth aspect of the present invention, there is provided a method ofdetecting pressure distribution in a fluid using a waveguide adapted todeform asymmetrically transverse to its length on exertion ofsubstantially isotropic pressure by the fluid such that thebirefringence of the waveguide varies with exerted pressure, but remainslow over the range of fluid pressures in which it is intended to use thewaveguide. Furthermore, according to a fifth aspect of the presentinvention, there is provided a method of manufacturing an opticalwaveguide for use in detecting pressure distribution in a fluid, thewaveguide being adapted to deform asymmetrically transverse to itslength on exertion of substantially isotropic pressure by the fluid suchthat birefringence of the waveguide varies with the exerted pressure,the method comprising selecting the birefringence of the waveguide toremain low over the range of fluid pressures in which it is intended touse the waveguide.

This can be achieved in several ways. For example, the optical waveguidemay have low intrinsic birefringence. This can help to keep thebirefringence of the optical fibre low in the presence of inducedbirefringence. The birefringence of the waveguide typically comprisesjust the sum of the intrinsic birefringence of the waveguide, which isdetermined at a neutral pressure, and induced birefringence caused bypressure exerted on the waveguide by the fluid. The waveguide maytherefore be arranged such that birefringence induced by increasingfluid pressure opposes (the) intrinsic birefringence of the waveguide.In other words, the induced birefringence can oppose the intrinsicbirefringence as the pressure increases from the neutral pressure. Thismeans that the induced birefringence can act say to reduce thewaveguide's birefringence from a given value of intrinsic birefringenceas the pressure exerted by the fluid increases. In other words, inducedbirefringence might subtract from intrinsic birefringence.

The applicants have also recognised that careful selection of the levelof intrinsic birefringence and the sensitivity of the fibre's inducedbirefringence can allow a pressure range over which the waveguide iseffective for detecting pressure to be selected as desired. Inparticular, a value of intrinsic birefringence can be selected such thatthe waveguide has substantially zero birefringence at a pressure offsetfrom atmospheric pressure to define a range of fluid pressuresdetectable using the waveguide. Similarly, the deformability of thewaveguide can be selected to control the rate of change of inducedbirefringence with increasing fluid pressure and thereby to define a/therange of fluid pressures detectable using the waveguide.

Birefringence can be quantified by birefringence beat length L_(B),which is the length of a waveguide over which the retardance between twoorthogonal eigenmodes of polarised light becomes 2π rad. A highbirefringence optical fibre typically has birefringence beat lengthbelow 10 cm. A moderate birefringence optical fibre typically hasbirefringence beat length between 10 cm and 1 m. A low birefringenceoptical fibre typically has birefringence beat length over 1 m. It istherefore preferred that the optical waveguide of the invention hasbirefringence beat length of more than around 1 m over the range offluid pressures in which it is intended to use the waveguide. Indeed, itis preferred that the intrinsic birefringence beat length of thewaveguide is longer than around 10 m.

As mentioned above, the length of the light pulses that can be used toobtain useful polarisation information from the waveguide is related tothe birefringence of the waveguide. More specifically, the applicantshave identified that the birefringence beat length should be at leasttwice as long as the light pulse so that the detected polarisationstates contain a strong indication of the birefringence of thewaveguide. Expressed differently, the spatial length of the light pulseis preferably less than around ½ of the minimum expected birefringencebeat length of the waveguide due to both intrinsic and inducedbirefringence over the pressure range in which it is intended to use thewaveguide. On the other hand, the light pulse should be long enough tomake the polarisation states practically measurable. So, the spatiallength of the light pulse is preferably between around ¼ to ½ of theminimum expected birefringence beat length of the waveguide. Even morespecifically, it is ideally around ½ the minimum expected beat length ofthe waveguide. Typically, the minimum expected birefringence beat lengthmight again be around 1 m.

The optical waveguide may take a variety of forms, but it is typicallyan optical fibre. This usually comprises a cladding and core ofdifferent refractive indices, e.g. made from silica. Alternatively, theoptical waveguide may be a photonic crystal fibre (PCF). PCFs comprisean array of cavities extending along the length of the fibre thatdiffract light in a controlled manner to guide light along the length ofthe fibre.

Similarly, the waveguide may be adapted to deform asymmetricallytransverse to its length on application of substantially isotropicpressure by the fluid in an almost endless variety of ways. However, inone preferred example, the waveguide has one or more holes extendingalong its length that make the waveguide more compressible in onedirection across its length than others. Preferably the waveguide hastwo such holes arranged on opposite sides of a core. This should resultin the core being compressed less in the direction extending between theholes than in other directions on application of external isotropicpressure. In other words, it is preferred that the optical waveguide isa side-hole optical fibre.

In the case of a side-hole fibre, the induced birefringence caused byexternal isotropic pressure can be determined by the dimensions andpositioning of the holes. In particular, it is preferred that, wherethere are two holes, one on either side of the core, the angle subtendedby the radius of each hole around the centre of the core should be lessthan around 10°. Such a fibre is suitable for use in pressure rangesbetween around 100 bar and around 2000 bar. For pressure ranges betweenaround 10 bar and around 100 bar, the angle need only be less thanaround 25°.

In another example, the optical waveguide may comprise an optical fibrewith a cladding that is substantially cylindrical apart from a segmentthat is effectively cut away (e.g. not present, but not necessarilyremoved by cutting) on one side of the fibre, e.g. so that there is asubstantially flat surface on one side of the fibre. In other words, theoptical waveguide may be a D-shaped optical fibre. Like side-holeoptical fibres, D-shaped optical fibres deform asymmetrically transverseto their length on application of substantially isotropic pressure.

The invention has a vast number of actual and potential applications.For example, the detection of fluid pressure is very important indown-holes of oil wells. The optical waveguide can therefore be placedin a down-hole of an oil well to detect pressure distribution of fluid,e.g. oil and/or water, in the down-hole. By calibrating the relationbetween waveguide birefringence and fluid pressure appropriately andcontinually monitoring birefringence along the length of the waveguide,the actual fluid pressure along the down-hole can be monitored.

In other examples, the optical waveguide can be placed in a variety ofother fluid pipes and such like to monitor pressure. In particular, theoptical waveguide might be placed in a water main, where changes inpressure along the length of the water main can be indicative ofblockages and leaks.

In yet another example, the fluid flow can be detected. An apparatus fordetecting fluid flow might comprise: the apparatus described abovewherein the derived indication detects the distribution along thewaveguide of the pressure exerted by the fluid on the waveguide; andmeans for distinguishing a component of the indication resulting fromstatic fluid pressure from a component of the indication resulting fromfluid flow. Similarly, a method of detecting fluid flow might comprise:the method described above wherein the derived indication detects thedistribution along the waveguide of the pressure exerted by the fluid onthe waveguide; and distinguishing a component of the indicationresulting from static fluid pressure from a component of the indicationresulting from fluid flow. Indeed, this is considered to be new initself and, according to a sixth aspect of the present invention, thereis provided an apparatus for detecting fluid flow, the apparatuscomprising:

an optical waveguide for extending through the fluid;

means for deriving an indication of the distribution along the waveguideof pressure exerted by the fluid on the waveguide; and

means for distinguishing a component of the indication resulting fromstatic fluid pressure from a component of the indication resulting fromfluid flow.

Also, according to a seventh aspect of the present invention, there isprovided a method of detecting fluid flow using an optical waveguideextending through the fluid, the method comprising:

deriving an indication of the distribution along the waveguide ofpressure exerted by the fluid on the waveguide; and

distinguishing a component of the indication resulting from static fluidpressure from a component of the indication resulting from fluid flow.

In other words, the variation in pressure along the length of theoptical waveguide can be determined, e.g. using the method describedabove. It is then possible to distinguish the portion of this variation,if any, that is due to static pressure from the portion that is due todynamic pressure or fluid flow. This is possible because the portion ofthe pressure variation that results from static pressure has predictablecharacteristics.

For example, the optical waveguide may be arranged to extend across theheight of the fluid (e.g. from at least one height to another height andpreferably across most or all of the height of the fluid). Thedistinguishing means may then comprise means for identifying variationsin pressure uncharacteristic of changing height. More specifically, aspressure changes linearly with fluid height, deviations from this linearincrease can be attributed to fluid flow reliably (provided any likelyother factors are taken into account). So, in one example, thedistinguishing means can subtract a linear component of the indicationto leave the component resulting from fluid flow. In another example,changes in the rate of change of the pressure distribution along thewaveguide can be identified as boundaries of fluid flow.

Alternatively the optical waveguide can be arranged to extendsubstantially horizontally in the fluid. This means that there should besubstantially no difference in static pressure along the length of thewaveguide. So, the distinguishing means may then comprise means forholding the waveguide horizontally in the fluid. Any variation inpressure along the waveguide can then be attributed to fluid flow(provided any likely other factors are taken into account).

Of course, detecting the pressure distribution along a single waveguidetypically only provides a one-dimensional pressure distribution, e.g. inthe direction that the waveguide extends. Should it be desired to mappressure in two dimensions, an optical waveguide grid may be used. Theapparatus preferably therefore comprises an optical waveguide grid forextending through the fluid. The grid might comprise a single opticalwaveguide bent to cross back and forth over itself, e.g. substantiallyat right angles. This allows a single pressure indication derivationmeans (e.g. light source, polarisation detector and processor) to beused. In another example, the waveguide grid may comprise severalwaveguides arranged to cross one another, e.g. substantially at rightangles. The grid (or individual parts or waveguides of the grid) can bearranged to extend across the height of the fluid or horizontally, asdescribed above in relation to a single waveguide above, to account forthe contribution of static pressure to the derived pressure indicationand allow detection of fluid flow.

The distinguishing means can take a variety of forms. In astraightforward example, the distinguishing means might be a display,such as a computer monitor, that is able to display a graphicalrepresentation of the pressure distribution. However, distinguishing thecomponent of the indication resulting from static fluid pressure fromthe component of the indication resulting from fluid flow lends itselfto automation and the distinguishing means may therefore be a processorrunning appropriate software or such like.

In another example, an interface between two fluids of different densitymay be the located. An apparatus for locating of an interface betweentwo fluids of different density might comprise the apparatus describedabove, wherein the waveguide is able to extend through the fluids, andmeans for locating a change in the gradient of the detected pressuredistribution along the waveguide to locate the fluid interface.Likewise, a method of locating of an interface between two fluids ofdifferent density might comprise the method described above, wherein thewaveguide extends through the fluids and locating a change in thegradient of the detected pressure distribution along the waveguide tolocate the fluid interface. Indeed, this is considered to be new initself and, according to an eighth aspect of the present invention,there is provided an apparatus for locating of an interface between twofluids of different density, the apparatus comprising:

an optical waveguide for extending through the fluids;

a detector for detecting a distribution along the waveguide of pressureexerted by the fluids on the waveguide; and

means for locating a change in the pressure gradient along the waveguideto locate the fluid interface.

Also, according to a ninth aspect of the present invention, there isprovided a method of locating of an interface between two fluids ofdifferent density using an optical waveguide extending through thefluids, the method comprising:

detecting a distribution along the waveguide of pressure exerted by thefluids on the waveguide; and

locating a change in the pressure gradient along the waveguide to locatethe fluid interface.

So, the distribution of pressure along the length of the waveguide canbe used to identify the interface between two fluids of differentdensity. This is possible because pressure change with fluid depth isproportional to fluid density. The rate of change of pressure in the twofluids will therefore be different. Variation in the rate of change ofpressure along the waveguide can therefore be used to identify andlocate the fluid interface.

Of course, where there are more than two fluids of different densitypresent, multiple such interfaces can be located. However, these aspectsof the invention are primarily concerned with the location of a singleinterface. Typically, the fluids are immiscible, as immiscible fluidstend to have a clearly defined interface between one another. Forexample, the fluids might be oil and water in a cavity. In otherexamples, the fluids might just have a tendency to separate. For examplewater or oil containing a suspension of solid particles can settle intotwo or more layers of different density fluids, e.g. water and slurry.

A particular application of this aspect of the invention is estimatingthe amount of oil left in a reservoir, e.g. a natural subterranean oilreservoir. This might involve estimating the volume of the reservoir,e.g. by seismic surveying, and determining the depth of oil in thereservoir using the invention. Effective oil volume estimation allowsfor efficient reservoir management and control, leading to optimisationof oil yield and production rates. This is extremely valuable in oilextraction.

The locating means can take a variety of forms. In a straightforwardexample, the locating means might be a display, such as a computermonitor, that is able to display a graphical representation of thepressure distribution and, e.g., distance along the waveguide. However,locating the change in the pressure gradient along the waveguide lendsitself to automation and the locating means may therefore be a processorrunning appropriate software or such like.

In another example, a tsunami may be detected. An apparatus fordetecting a tsunami may comprise the apparatus described above, whereinthe waveguide is able to extend underwater along an expected path of atsunami, the processor derives an indication of the distribution alongthe waveguide of the pressure exerted by the water on the waveguide andhaving means for determining when changes in the distribution arecharacteristic of a tsunami travelling through the water along theexpected path. Likewise, a method of detecting a tsunami may comprisethe method described above, wherein the optical waveguide extendsunderwater along the expected path of a tsunami, the derived indicationis of the distribution along the waveguide of the pressure exerted bythe water on the waveguide, and determining when changes in thedistribution are characteristic of a tsunami travelling through thewater along the expected path. Indeed, this is considered to be new initself and, according to a tenth aspect of the present invention, thereis provided an apparatus for detecting a tsunami, the apparatuscomprising:

an optical waveguide for extending underwater along an expected path ofa tsunami;

a detector for detecting a distribution along the waveguide of pressureexerted on the waveguide by the water;

and means for determining when changes in the detected pressuredistribution are characteristic of a tsunami travelling through thewater along the expected path.

Also, according to an eleventh aspect of the present invention, there isprovided a method of detecting a tsunami using an optical waveguideextending underwater along an expected path of a tsunami, the methodcomprising:

detecting a distribution along the waveguide of pressure exerted on thewaveguide by the water;

and determining when changes in the detected pressure distribution arecharacteristic of a tsunami travelling through the water along theexpected path.

So, by looking at the distribution of external isotropic pressure along(e.g. the length or the whole length of) the optical waveguide overtime, the propagation of a tsunami can be detected. These aspects of theinvention are particularly effective, as the waveguide can extend overseveral tens of kilometres. Not only does this allow remote detection ofthe tsunami, it can help to provide effective early warning, well beforea tsunami reaches a coast. Furthermore, changes in the distribution ofpressure exerted along the waveguide by a tsunami can provide usefulscientific data about the nature of tsunamis, not always possible withother systems that have shorter reach and/or restricted pressuremeasurement locations.

In this regard, the waveguide may of course be located at anyappropriate position along the expected path of a tsunami, e.g. close toa geographical fault line or such like. However, it is preferred thatthe waveguide is arranged to extend away from a coastline, e.g.substantially perpendicular to the coastline. This allows the detectorand determining means to be conveniently located on land. The long reachof the waveguide makes this possible.

Similarly, the waveguide may be positioned at any convenient depth. Thisis possible because the pressure fluctuation of a tsunami extendsthroughout the depth of the water in which they travel. In one example,buoys may be attached to the optical waveguide to support itsubstantially horizontally in the water (at the desired depth). Oneadvantage of this is that the effect of water depth on pressure alongthe length of the waveguide can be kept constant, which can make changesin pressure easier to measure. In another example, the waveguide may beadapted to extend along the seabed. This can make deployment andmaintenance of the waveguide more straightforward.

One waveguide may be sufficient to allow effective detection of atsunami. However, it is again preferred that there are severalwaveguides. Each waveguide may have its own detector (e.g. light source,polarisation detector and processor). However, the determining means maydetermine when changes in the detected pressure distributions of morethan one, e.g. all, of the waveguides are characteristic of a tsunamitravelling through the water along the expected path. In other words,the determining means may compare the detected pressure distributions ofthe waveguides.

Tsunamis typically have wavelengths approaching a few kilometres in deepwater, which makes them hard to detect. For this reason, it is importantfor the determining means to look at the distribution of pressureexerted along the waveguide. More specifically, the determining meanscan look at changes in the pressure distribution that move along thelength of the waveguide. In particular, the determining means maydetermine that changes in the detected pressure distributions of morethan one waveguide are characteristic of a tsunami travelling throughthe water when each distribution includes a wave of increased pressuremoving along the length of the respective waveguide at approximately thesame speed. This is particularly useful in eliminating spurious waveevents.

Use of the word “processor” is intended to be general rather thanspecific. Whilst the invention may be implemented using an individualprocessor, such as a central processing unit (CPU), it could equallywell be implemented using other suitable components or apparatus. Forexample, the invention could be implemented using a hard-wired circuitor circuits, e.g. an integrated circuit, or using embedded software. Itcan also be appreciated that the invention can be implemented, at leastin part, using computer program code. According to a twelfth aspect ofthe present invention, there is therefore provided computer software orcomputer program code adapted to carry out the method described abovewhen processed by a processing means. The computer software or computerprogram code can be carried by computer readable medium. The medium maybe a physical storage medium such as a Read Only Memory (ROM) chip.Alternatively, it may be a disk such as a Digital Video Disk (DVD-ROM)or Compact Disk (CD-ROM). It could also be a signal such as anelectronic signal over wires, an optical signal or a radio signal suchas to a satellite or the like. The invention also extends to a processorrunning the software or code, e.g. a computer configured to carry outthe method described above.

Similarly, use of the word “fluid” refers to both liquids and gasesunless otherwise qualified. More specifically, the invention can be usedto measure pressure distribution in both gases and liquids or, indeed,any medium capable of exerting isotropic pressure on a waveguide.

Preferred embodiments of the invention are now described, by way ofexample only, with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a pressure sensing apparatusaccording to the invention;

FIG. 2 is a schematic illustration showing a cross section of aside-hole optical fibre suitable for use in the present invention;

FIG. 3 is a graphical illustration of birefringence of the optical fibreillustrated in FIG. 2 versus pressure exerted on the fibre;

FIG. 4 is a schematic illustration showing a cross section of anotherside-hole fibre suitable for use in the present invention;

FIG. 5 is a graphical representation of birefringence of the opticalfibre illustrated in FIG. 4 versus pressure exerted on the fibre;

FIG. 6 is a graphical representation of birefringence of variations ofthe optical fibre illustrated in FIG. 4 versus pressure exerted on thefibre;

FIG. 7 is a schematic illustration showing a cross section of a D-shapeoptical fibre suitable for use in the present invention;

FIG. 8 is a longitudinal, sectional view of an optical fibreillustrating operation of the apparatus of FIG. 1 for measuringpressure;

FIG. 9 is a schematic illustration of an optical fibre disposed formeasuring fluid flow, along with a graphical representation of pressureagainst fluid depth;

FIG. 10 is a schematic illustration of an optical fibre disposed forlocation of an oil/water interface, along with a graphicalrepresentation of pressure against fluid depth;

FIG. 11 is a schematic illustration of an optical fibre disposed fortsunami detection, along with a graphical representation of pressureagainst position along at three distinct times.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a pressure sensing apparatus 100 comprises a lightsource 102 which, in this embodiment, is a tunable laser able totransmit coherent light across a range of wavelengths between around1550 nm and 1560 nm. (In other embodiments, any range of wavelengthsbetween about 1300 nm and 1600 nm could be used.) Light transmitted bythe light source 102 is directed into a polarisation processing unit(PPU) 104. The PPU 104 is operable to condition the polarisation stateof the light received from the light source 102 and to transmit some ofthe light incident on it from the light source 102 to an optical coupler106.

The optical coupler 106 is attached to an end 112 of an optical fibre110 and directs light from the PPU 104 into the optical fibre 110through the end 112 of the optical fibre 110 to which it is attached.The optical coupler 106 also transmits light emitted from the end 112 ofthe optical fibre 110, for example by backscattering in the opticalfibre 110, to the PPU 104. The polarisation state of this emitted, e.g.backscattered, light is analysed by the PPU 104 in combination with thephoto detector 116 and a processor 118. More specifically, the PPU 104is able to separate the light emitted from the end 112 of the opticalfibre 110 into four components, each of which define the intensity oflight polarised in a particular direction. These four components definethe polarisation state of light in the Stokes formalism and are outputby the PPU 104 to the photo detector 116, which in this example is aphoto diode array. The photo detector 116 is able to detect theintensity of the light in each of the components isolated by the PPU 104and output data defining the detected light intensities to the processor118.

In this embodiment, the processor 118 is the central processing unit(CPU) of a personal computer (PC) and is connected to a display 120,which, in this embodiment, is a monitor of the PC. The processor 118 isable to analyse the data output by the photo detector 116 and derive thebirefringence b(z) of the optical fibre 110 (where z is a vector alongthe length of the fibre 110), as set out in more detail below, andoutput the results to the display 120.

The processor 118 is also connected to a light source and PPU controller122 for controlling the light source 102 and PPU 104. The light sourceand PPU controller 122 is operable to select: the wavelength of lightemitted by the light source 102; light pulse timing; and light pulseduration, along with the polarisation state of light emitted by the PPU104. Control of the timing and duration of the light pulses emitted bythe light source 102, in combination with the timing of the output ofthe photo detector 116 allows the processor 118 to distinguish thepolarisation state of light backscattered in different parts of theoptical fibre 110. In other words, backscattering in different sectionsof the optical fibre 110 can be resolved by the processor 118.

The fibre 110 has birefringence b, which is equivalent to the differenceΔn between the refractive indices n of the two polarisation eigenmodesof light propagated by the fibre 110. More specifically, in asubstantially neutral pressure environment, e.g. at atmosphericpressure, the fibre 110 has intrinsic birefringence b₀ resulting fromits inherent structure. When the fibre 110 has isotropic externalpressure exerted on it, e.g. when it is immersed in a fluid, the fibre'score experiences asymmetric strain. This asymmetric strain causes thebirefringence b of the fibre 110 to change, e.g. for the fibre 110 toexperience induced birefringence b_(i).

Birefringence b is strictly a dimensionless quantity. However, it oftenexpressed as a value of rad/m for a given wavelength of light. This isparticularly common in relation to monomode optical fibres, as thesefibres are designed predominantly to carry light at only a singleoperating wavelength λ (in this embodiment around 1550 nm to 1560 nm)and the quoted value of birefringence b is implicitly based on thisoperating wavelength λ. Alternatively, birefringence b can be expressedas a value of birefringence beat length L_(b), which is linked to theoperating wave length λ by the relation $L_{B} = \frac{\lambda}{b}$

In order to extract a backscatter polarisation signal that can be usedto determine the value of the local beat length L_(b) of the fibre 110,the local beat length L_(b) of the fibre 110 should ideally be at leasttwice the spatial length of the light pulse used to interrogate thefibre 110. For a light pulse with time duration τ, this condition can beexpressed as ${\frac{c}{n}\tau} \leq \frac{L_{B}}{2}$

where c is the speed of light and n is average isotropic refractiveindex of the fibre's core. This condition should hold across the rangeof pressures intended to be measured by the fibre 110.

Generally, the duration τ of the light pulse is as short as possible inorder to maximise spatial resolution along the length of the fibre 110of the signal generated by the photodetector 116 from lightbackscattered in the fibre 110 from the light pulse. However, theduration τ of the light pulse must be long enough that sufficient lightis backscattered in the fibre 110 to be practically detectable by thephotodetector 116. This should ideally be achieved without the lightpulse having intensity great enough to cause unwanted and confusingnon-linear effects in the backscatter process. In other words, theduration τ of the light pulse is limited to being above a given value,e.g. of the order of nanoseconds, or longer than a given spatial length,e.g. of the order of 10s of centimetres, so that a useful backscattersignal can be detected by the photodetector 116. So, importantly, thereis a maximum birefringence b_(max) measurable by this technique, whichcan be expressed as a minimum birefringence beat length L_(b), and isaround a level usually referred to as “low birefringence”.

In one embodiment, the optical fibre 110 shown in FIG. 1 is a side-holefibre 200 as shown in cross section in FIG. 2. This side-hole fibre 200comprises a cladding 202 and a central core 204. Both of these aresubstantially cylindrical. On opposite sides of the core 204 are twoholes 206 that extend parallel to the core 202 along the length of thefibre 200. In this embodiment, the holes 206 contain air, e.g. they are“air holes”. In other embodiments, they may contain other gases or evensolids or liquids, provided they are much more compressible than thematerial (e.g. silica) of the cladding 202. (In yet other embodiments,the material in the holes 206 may be less compressible than the materialof the cladding, although this of course alters the orientation of theasymmetric compressibility of fibre 200 described below.) The holes 206each have radius r. The distance d from the centre of the core 204 tothe centre of each hole 206 is the same for each hole 206. Similarly, anangle Φ subtended by the radius r of each hole 206 about the centre ofthe core 204 (in other words the angle the between a straight linejoining the centre of the core 204 and the centre of a respective hole206 and a straight line joining the centre of the core 204 and thetangent of the hole 206) is the same for each hole 206. The presence ofthe holes 206 means that the core 204 of the fibre 200 experiences lessstress in a direction (orthogonal or transverse to its length) thatextends between the holes 206 than in other directions (orthogonal ortransverse to its length) when isotropic external pressure is applied tothe fibre 200. The core 204 therefore experiences less strain in thatdirection than others. In other words, the core 204 of the fibre 200experiences asymmetric strain under the influence of external isotropicpressure.

In this embodiment, the intrinsic birefringence b₀ of the fibre 200 islow, very low or even negligible. In addition, the amount by which thebirefringence b of the fibre 200 changes with applied external pressureis restricted. More specifically, the induced birefringence b_(i) of thefibre 200 is kept low over the range of fluid pressures in which it isintended to use the fibre 200.

So, referring to FIG. 3, when the fibre 200 is in a neutral pressureenvironment, referred to as a minimum pressure P_(min), the fibre 200has only its low intrinsic birefringence b₀, illustrated as zerobirefringence in FIG. 3. As the fibre 200 has increasing externalisotropic pressure exerted on it, it experiences induced birefringenceb_(i), illustrated as negative in FIG. 3, which causes the birefringenceb of the fibre 200 to change linearly with pressure. This inducedbirefringence b_(i) is detectable until it reaches a (negative) maximumbirefringence b_(max). The maximum birefringence b_(max) corresponds toa maximum pressure P_(max) detectable by the fibre 200. So the fibre 200is useful for measuring pressure in range extending from a minimumpressure P_(min) to a maximum pressure P_(max).

The rate of change of induced birefringence b_(i) with exerted pressureand hence the maximum pressure P_(max), corresponding to the maximumbirefringence b_(max) of the fibre 200, can be controlled using asuitable fibre construction. In particular, the dimensions andpositioning of the holes 206 of the optical fibre 200 are carefullychosen. This can allow the strain experienced by the core 204 of thefibre 200 to be limited over the range of fluid pressures P_(min) toP_(max) in which it is intended to use the fibre 200. Limiting thestrain experienced by the fibre 200 limits the induced birefringenceb_(i) of the fibre 200 at a given fluid pressure.

Table 1 below shows the range of fluid pressures P_(min) to P_(max) thatcan be measured using variations of the optical fibre 200 with holes 206having different radii r and distances d from the centre of the core204. In this example, the light pulses have duration τ of 1 ns andwavelength λ of 1550 nm; and the fibre 200 has intrinsic birefringencebeat length of approximately 18.7 m, which is considered as negligiblefor simplicity here. The optical fibre 200 has a pure fused silicacladding and a lightly doped germanium core 204. It also has an overalldiameter of around 125 μm and a core diameter of between around 8 μm to10 μm. TABLE 1 Pressure Range d (μm) r (μm) (bar) 30 15 10 30 9 30 30 5101 40 5 180 45 5 230 50 4 442 50 3 786 50 2 1770

So, broadly, the angle Φ subtended by the radius r of each hole 206about the centre of the core 204 is less than around 10° for pressureranges between around 100 bar and around 2000 bar and less than around25° for pressure ranges between around 10 bar and around 100 bar.

However, one difficulty with this approach is that, as it is desired tomeasure increasingly high fluid pressures, the sensitivity of themeasurement decreases. More specifically, the maximum birefringenceb_(max) that can be detected is fixed by the duration τ of the lightpulse used to interrogate the fibre 200, as discussed above. Thepressure range P_(min) to P_(max) detectable by the fibre 204 istherefore always effectively scaled across a fixed range ofbirefringence b, below this maximum birefringence b_(max). As thepressure range P_(min) to P_(max) increases, the pressure changerepresented by a given change in birefringence b therefore increases. Asthe minimum pressure P_(min) is fixed by the intrinsic birefringence ofthe fibre 200, higher pressures can only be measured with lowersensitivity.

So, in another embodiment, referring to FIG. 4, the intrinsicbirefringence b_(o) of an optical fibre 400 is arranged to have oppositesign to the induced birefringence b_(i) of the fibre 400 as the pressureexerted on the fibre 400 increases and its value is selected inaccordance with the range of fluid pressures P_(min) to P_(max) overwhich it is intended to use the fibre. The construction of the opticalfibre 400 illustrated in FIG. 4 is similar to the construction of theoptical fibre 200 illustrated in FIG. 2. It is a side-hole fibre 400having a cladding 402 and a central core 404. The cladding 402 issubstantially cylindrical. However, the core 404 is substantiallyelliptical, rather than cylindrical. This means that the core 404 canhave significantly different refractive indices n of the twopolarisation eigenmodes of light propagated by the fibre 400 at neutralpressure. In other words, the fibre 400 can have appreciable intrinsicbirefringence b₀, as described in more detail below.

Similarly to the optical fibre 200 illustrated in FIG. 2, the opticalfibre 400 illustrated in FIG. 4 has holes 406 on each side of the core404 that extend parallel to the core 402 along the length of the fibre400. Again, the holes 406 can be air holes or they may contain othergases or even solids or liquids, as desired. The holes 406 each haveradius r; the distance d from the centre of the core 404 to the centreof each hole 406 is the same for each hole 406; and the angle Φsubtended by the radius r of each hole 406 about the centre of the core404 is the same for each hole 406. The dimensions and positioning of theholes 406 can be adjusted to vary the amount of strain experienced bythe core 404 of the fibre for a given pressure in a similar way to theoptical fibre 200 illustrated in FIG. 2.

Referring to FIG. 5, it can be appreciated that the intrinsicbirefringence b_(o) of the optical fibre 400 having opposite sign to theinduced birefringence b_(i) can allow the birefringence of the opticalfibre 400 to decrease as the pressure exerted on it increases. If theintrinsic birefringence b₀ of the fibre 400 is higher than the (modulusof) maximum birefringence b_(max), pressures close to the neutralpressure of the fibre 400, e.g. close to atmospheric pressure are notdetectable. However, as the pressure increases, the inducedbirefringence b_(i) of the fibre 400 subtracts from the intrinsicbirefringence b₀ of the fibre 400 and the birefringence b of the fibre400 decreases. Once a pressure at which the birefringence of the fibre400 decreases below the maximum birefringence b_(max), the birefringenceof the fibre 400 becomes detectable. However, the technique of detectingfibre birefringence b described in this document actually measures themodulus of the fibre's birefringence b. It is therefore preferred thatthe fibre 400 is used to detect a range of pressures P_(min) to P_(max)for which the minimum pressure P_(min) corresponds to substantially zerobirefringence. This avoids ambiguity in the determination of pressure.So, referring again to FIG. 5, the range of pressures P_(min) to P_(max)detectable by the fibre 400 extends from a minimum pressure P_(min) atwhich the sum of the fibre's intrinsic birefringence b₀ and the fibre'sinduced birefringence b_(i) is substantially zero and a maximum pressureP_(max) at which the sum of the fibre's intrinsic birefringence b₀ andthe fibre's induced birefringence b_(i) is substantially equal to the(negative or modulus) maximum measurable birefringence b_(max) of thefibre 400.

In further embodiments, careful selection of the intrinsic birefringenceb₀ of the optical fibre 400 and the rate of change of inducedbirefringence b_(i) with pressure can allow the optical fibre 400 to beused to measure pressure over a variety of different pressure rangesP_(min) to P_(max). Generally, the birefringence of the fibre 400 can beexpressed as a function of the isotropic pressure exerted on the fibre400, e.g.b(P)=α(P−P _(min))

where α is a birefringence pressure sensitivity coefficient for theoptical fibre 400, determined by the size and positions of the holes 406of the fibre for example. The sensitivity coefficient α can be expressedin terms of the maximum detectable fibre birefringence b_(max) and thedesired pressure range P_(min) to P_(max), e.g.$\alpha = \frac{b_{\max}}{\left( {P_{\max} - P_{\min}} \right)}$

In this Embodiment, the Sensitivity Coefficient α, Rate of Change ofinduced birefringence b_(i) with isotropic pressure exerted on the fibre400 or, broadly, fibre “sensitivity” can be varied by changing thedimensions and positions of the holes 406, as described in relation tothe optical fibre 200 illustrated in FIG. 2. The intrinsic birefringenceof the optical fibre 400 can be expressed in terms of the sensitivitycoefficient α and the desired minimum detectable pressure (or pressureat which the sum of the fibre's intrinsic birefringence b₀ and thefibre's induced birefringence b_(i) is substantially zero), e.g.|b _(o) =|αP _(min)|

The level of intrinsic birefringence b₀ of the fibre 400 can be variedby varying the ellipticity of the core. The typical values of intrinsicbirefringence b₀ required are less then those current commerciallyavailable elliptical core optical fibres, e.g. less then around 2×10⁻⁴.The ratio of ellipticity of the core 404 is therefore likely to be lessthan in those fibres, e.g. less than around 1:2.5. The level ofintrinsic birefringence b₀ of the fibre 400 can also be varied byaltering the materials from which the optical fibre 400 is made and, inparticular, the doping of the core 404. Again, doping the core 404 tohave a lower refractive index than levels than most current commerciallyavailable elliptical core optical fibres is likely to be appropriate.

Some examples of different pressure ranges P_(min) to P_(max) areillustrated in FIG. 6. Line A represents a variation of optical fibre400 having an intrinsic birefringence b₀ and sensitivity coefficient αchosen such that the fibre 400 has a pressure range P_(min) to P_(max)from 200 bar to 1000 bar. Line B represents a variation of optical fibre400 having an intrinsic birefringence b₀ and sensitivity coefficient αchosen such that the fibre 400 has a pressure range P_(min) to P_(max)from 800 bar to 1000 bar. Line C represents a variation of optical fibre400 having an intrinsic birefringence b₀ and sensitivity coefficient αchosen such that the fibre 400 has a pressure range P_(min) to P_(max)from 200 bar to 600 bar. Line d represents a variation of optical fibre400 having an intrinsic birefringence b₀ and sensitivity coefficient αchosen such that the fibre 400 has a pressure range P_(min) to P_(max)from 400 bar to 600 bar. In each of these examples, the maximumdetectable fibre birefringence b_(max) is taken as around 3.9×10⁻⁶,which corresponds to a birefringence beat length L_(b) of around 40 cmat an operating wavelength λ of around 1550 nm.

In another example, the optical fibre 110 shown in FIG. 1 is a lowbirefringence D-shaped fibre 700 as shown in FIG. 7. The D-shaped fibre700 has a cladding 702 and a core 704. The core 704 is cylindrical (orelliptical) and the cladding 702 extends around the core 704. So, theinside surface of the cladding 702 is cylindrical (or elliptical). Inother words, the core-cladding interface has circular (or elliptical)cross-section. However, whilst the majority of the outer wall of thecladding 704 is cylindrical, a section of the cladding 702 iseffectively cut away. This leaves a flat surface 706 on one side of theoptical fibre 700 and the optical fibre 700 is narrowest in a directionx-y perpendicularly bisecting the surface 706 along its width andpassing through the centre of the core 704. The D-shaped fibre 700 istherefore more compressible in the direction x-y than in otherdirections. This makes it similar to the side-hole fibres 200; 300 inthat, when the D-shaped fibre 700 experiences isotropic externalpressure, the core 704 experiences less strain in the direction x-y thanin other directions.

The D-shaped fibre 700 has similar constraints on its birefringence tothose of the side-hole fibres 200; 300. In other words, it shouldideally have sufficiently low intrinsic birefringence to allowresolution of its birefringence properties using light pulses ofduration that are practically producible and detectable; and the localbirefringence that it experiences with applied external pressure must besufficiently small that this remains that case over the pressure rangeit is intended to measure. However, the intrinsic birefringence andsensitivity of the optical fibre 700 can be selected in similar ways tothe optical fibres 200; 400 illustrated in FIGS. 2 and 4 to select apressure range for the optical fibre 700, as desired.

Of course, other suitable optical fibre designs will occur to theskilled person. For example, the optical fibre 110 may be a photoniccrystal fibre (PCF) rather than having a conventional silica core andcladding. Likewise, the optical fibre 110 may be adapted to deformasymmetrically transverse to its length on application of substantiallyisotropic pressure in an almost endless variety of ways.

Pressure Measurement

In use, the optical fibre 110 is first calibrated in a neutral pressureenvironment, e.g. in air at atmospheric pressure. This involves mappingthe intrinsic birefringence of the fibre 110 along the length of thefibre 110 that it is intended to use for pressure measurement. The fibre110 can then be immersed in a fluid (not shown), the pressure of whichit is desired to measure. In most cases the fluid is a liquid such aswater or oil, but the invention is not limited to measurement ofpressure in these liquids. Pressure in gases or liquid suspensions canalso be measured.

The birefringence measurement which is made is that of the spatialdistribution of linear retardance δ per unit length z, i.e. δ(z). Thisis related to the birefringence b(z) by${\delta(z)} = {\frac{2\pi}{\lambda}{b(z)}}$

where λ is the wavelength of the light pulse.

For operation at a single wavelength λ, δ(z) can be calculated byemploying two input polarization states; for example, two linear statesseparated by an angle of 45°. The reason for this is that if one of thelocal propagating states is linear and lies along one of the axes of alinearly birefringent local section of the fibre 110, no information onthe retardance δ between the eigenmodes is available. In this case, theinput polarization state orientated at 45° will provide a local linearpolarisation state now at 45° to the local birefringence axis of thesection, and the required information is recoverable. So, the PPU 104transmits two light pulses, each linearly polarised at 45° to oneanother.

Referring to FIG. 8, at time t the light backscattered from thepropagating light pulse returns from point A in the fibre 110. Thelength of fibre 110 up to point A is equivalent, polarimetrically, to aretarder/rotator pair. The retardation of the equivalent linear retarderis δ_(e)(z) and the orientation of its axes is q_(e)(z). The rotationinduced by the rotator is ρ_(e)(z). In backscatter, the rotator ρ_(e)(z)is cancelled. Consequently, at successive values of t, e.g. t+τ (z and tare related by 2z=ct, in backscatter) what are effectively measured areδ_(e)(t) and q_(e)(t), with a temporal/spatial resolution determined bythe width of the optical pulses and the response time of thephotodetector 116, whichever is the longer.

As mentioned above, the PPU 104 and photodetector 116 actuallyphysically measure the Stokes parameters of the backscattered light, asa function of time, for light pulses having the two input polarizationstates. The Stokes parameters define the ‘instantaneous’ polarizationstate of the backscattered light. This information allows δ(z) to becalculated from the values of the Stokes parameters and their timederivatives. δ(z) can then be related directly to the distribution ofpressure along the fibre 110.

Flow Measurement

According to Bernoulli's law, pressure P in a moving fluid can beexpressed as$P = {P_{0} + {\rho\quad{gh}} + {\frac{1}{2}\rho\quad v^{2}}}$

where P₀ is static pressure, ρ is fluid density, g is gravitationalacceleration, h is the height of fluid above a reference surface and υis the local fluid velocity at the position where the pressure isestimated. So, for example, an underwater current with a speed around 5m/s may create a dynamic pressure of 0.125 bar in addition to the staticpressure at the depth of the current. Fluid flow can therefore bedetermined by measuring isotropic pressure.

In order to determine fluid flow, the optical fibre 110 is positionedtransverse to the direction of fluid flow. Referring to FIG. 9, in oneembodiment, the fibre 110 is secured at one end to the seabed 900, forexample using a suitable weight or anchor (not shown), and the apparatus100 is mounted on a sea vessel or buoy (not shown) at the sea surface902. The apparatus 100 is used to derive a distribution of externalisotropic pressure along the length of the fibre 110 from the seasurface 902 to the seabed 900.

An example of the determined pressure distribution is shown in graphicalform in FIG. 9. As expected, the pressure increases proportionally withwater depth. However, where there are currents X, Y, Z in the water,i.e. the water is flowing; the pressure is increased out of proportionwith the corresponding depth. In particular, the boundaries of thecurrents X, Y, Z, are characterized by step-like changes in pressure.The depth of these boundaries is identified either by looking at thedistance along the fibre 110 at which they occur or by looking at thestatic pressure of the water in the region of the current. The dynamicpressure of the water in the currents X, Y, Z can also be used todetermine current velocity, using Bernoulli's law above.

In another embodiment (not shown), at least part of the optical fibre110 is held horizontally in the sea using buoys. This means that thestatic pressure of the water surrounding the part of the fibre 110 heldhorizontal is the same along the length of that part of the fibre 110.Any change in fibre birefringence b along that part of the fibre 110 cantherefore be attributed to dynamic pressure, i.e. currents in the water.

In yet another embodiment (not shown), the optical fibre 110 can bearranged as a grid or several optical fibres 110 can be used to form agrid. The grid can be laid so that the plane it forms is horizontal orvertical in the sea. This allows two dimensional imaging of seacurrents.

Location of an Oil/Water Interface

Referring to FIG. 10, a cavity or vessel, such as an oil reservoir,contains both oil 1000 and water 1002. As oil 1000 is lighter than water1002 and the two fluids are immiscible, the oil 1000 sits on top of thewater 1002. It can be useful to detect the level of oil 1000 in thereservoir, for example to aid extraction.

Fluid pressure increases linearly with depth according top=ρghwhere p is the isotropic fluid pressure, ρ fluid density, g isacceleration due to gravity and h is the depth below the surface. So,$\frac{\mathbb{d}p}{\mathbb{d}h} = {\rho\quad g}$This means that, when fluid density ρ changes abruptly, e.g. at anoil/water interface, the gradient of pressure with depth h changes. Thisis illustrated graphically on the right hand side of FIG. 10.

To locate an oil/water interface, optical fibre 110 is thereforeimmersed in the reservoir and apparatus 100 used to determine thedistribution of birefringence b along the length of the fibre 110. Bylooking at the gradient of the birefringence b along the length of thefibre 110, it is possible to identify a change in fluid density ρ andhence determine the depth of the oil/water interface.

Tsunami Detection

Tsunamis are large waves, most commonly generated by earthquakes thathave their epicentre undersea or by landslides into the sea. These wavescan hit coastlines with catastrophic consequences and it is generallydesirable to be able to detect the approach of tsunamis effectively witha view to providing early warning to coastal communities.

One important feature of tsunamis that distinguishes them from otherwater waves is that they comprise a significant fluctuation in pressurethroughout the depth of the water in which they travel. So, it ispossible to detect a tsunami using a pressure sensor placed underwateror on the seabed.

Accordingly, one or more optical fibres 110 are laid along the seabedextending out to sea from the coastline. In this embodiment, eachoptical fibre 110 has its own dedicated apparatus 100 that interrogatesthe fibre 110 periodically to determine the distribution ofbirefringence b along the length of the fibre 110. Changes in thedistribution of birefringence b can be indicative of changes in pressurealong the length of the fibre 110.

Referring to FIG. 11, a single optical fibre 110 is laid along theseabed 1102, with the detection apparatus 100 located at the coast. As atsunami travels through the water, an increase in water pressure occursalong the length of the fibre; leading to change in fibre birefringenceb detected by the apparatus 100. The change in fibre birefringence b atsuccessive times t₁, t₂ and t₃ is closer to apparatus 100 and hence thecoastline, indicating a tsunami travelling toward the coastline. Thestrength and speed of the tsunami can be derived from the measuredchange in fibre birefringence b.

In other embodiments, multiple fibres 110 can be used to characterisethe propagation of the wave in two dimensions. This can give moreinformation on the wave's direction and help to eliminate spurious waveevents.

The described embodiments of the invention are only examples of how theinvention may be implemented. Modifications, variations and changes tothe described embodiments will occur to those having appropriate skillsand knowledge. These modifications, variations and changes may be madewithout departure from the spirit and scope of the invention defined inthe claims and its equivalents.

1. An apparatus for detecting pressure distribution in a fluid, theapparatus comprising: an optical waveguide for submersion in the fluid,the optical waveguide being adapted to deform asymmetrically transverseto its length on exertion of substantially isotropic pressure by thefluid such that birefringence of the waveguide varies with the exertedpressure; a light source for directing pulses of light into thewaveguide; a polarisation detector for detecting polarisation states oflight backscattered from the light pulses in the waveguide; and aprocessor for deriving an indication of the birefringence of at leastpart of the waveguide from the detected polarisation states to detectpressure distribution in the fluid.
 2. The apparatus of claim 1, whereinthe optical waveguide is a side-hole optical fibre.
 3. The apparatus ofclaim 2, wherein the side-hole fibre has two holes, one on either sideof the fibre's core, and the angle subtended by the radius of each holearound the centre of the core is less than around 25°.
 4. The apparatusof claim 2, wherein the side-hole fibre has two holes, one on eitherside of the fibre's core, and the angle subtended by the radius of eachhole around the centre of the core is less than around 10°.
 5. Theapparatus of claim 1, wherein the optical waveguide is a D-shapedoptical fibre.
 6. The apparatus of claim 1, wherein the polarisationdetector is able to detect the polarisation state of light followingbackscattering at different points along the length of the waveguide andthe at least part of the waveguide for which the processor derives anindication of birefringence is between at least some of these points. 7.The apparatus of claim 1, wherein the processor derives an indication ofthe variation of birefringence along the length of the waveguide todetect spatial pressure distribution in the fluid.
 8. The apparatus ofclaim 1, wherein the processor derives an indication of thebirefringence of the at least part of the waveguide more than once todetect temporal pressure distribution in the fluid.
 9. The apparatus of,claim 1, wherein the optical waveguide can be placed in a down-hole ofan oil well and pressure distribution of fluid in the down-hole isdetected.
 10. The apparatus of, claim 1, wherein the optical waveguidecan be placed in a water pipe and pressure distribution of water in thepipe can be detected.
 11. An apparatus for detecting fluid flow, theapparatus comprising: the apparatus of claim 1 wherein the derivedindication detects the distribution along the waveguide of the pressureexerted by the fluid on the waveguide; and means for distinguishing acomponent of the indication resulting from static fluid pressure from acomponent of the indication resulting from fluid flow. 12-15. (canceled)16. An apparatus for locating an interface between two fluids ofdifferent density, the apparatus comprising the apparatus of claim 1,wherein the optical waveguide is able to extend through the fluids, andmeans for locating a change in the gradient of the detected pressuredistribution along the waveguide to locate the fluid interface. 17.(canceled)
 18. An apparatus for detecting a tsunami, the apparatuscomprising the apparatus of claim 1, wherein the optical waveguide isable to extend underwater along an expected path of the tsunami, theprocessor derives an indication of the distribution along the waveguideof the pressure exerted by the water on the waveguide and the apparatushas means for determining when changes in the distribution arecharacteristic of a tsunami travelling through the water along theexpected path. 19-24. (canceled)
 25. The apparatus of claim 1,comprising more than one waveguide.
 26. The apparatus of, claim 1,wherein the optical waveguide extends for several tens of kilometres.27. The apparatus of, claim 1, wherein the birefringence of the opticalwaveguide remains low over the range of fluid pressures in which it isintended to use the waveguide.
 28. The apparatus of claim 1, wherein theoptical waveguide has low intrinsic birefringence.
 29. The apparatus ofclaim 1, wherein the optical waveguide has intrinsic birefringence beatlength longer than around 10 m.
 30. The apparatus of claim 1, whereinthe waveguide is arranged such that birefringence induced by increasingfluid pressure opposes (the) intrinsic birefringence of the waveguide.31. The apparatus of claim 1, wherein the waveguide has a value ofintrinsic birefringence selected such that the waveguide hassubstantially zero birefringence at a pressure offset from atmosphericpressure to define a range of fluid pressures detectable using thewaveguide.
 32. The apparatus of claim 1, wherein the waveguide hasdeformability selected to control the rate of change of inducedbirefringence with increasing fluid pressure and thereby to define a/therange of fluid pressures detectable using the waveguide.
 33. Theapparatus of claim 1, wherein the waveguide comprises an optical fibrehaving an elliptical core.
 34. The apparatus of claim 1, wherein thebirefringence beat length of the optical waveguide remains above aroundtwo times the length of the light pulses over the range of fluidpressures in which it is intended to use the waveguide.
 35. An opticalwaveguide for use in detecting pressure distribution in a fluid, thewaveguide being adapted to deform asymmetrically transverse to itslength on exertion of substantially isotropic pressure by the fluid suchthat the birefringence of the waveguide varies with the exertedpressure, but remains low over the range of fluid pressures in which itis intended to use the waveguide.
 36. The waveguide of claim 35, whereinthe optical waveguide has low intrinsic birefringence.
 37. The opticalwaveguide of claim 35, having intrinsic birefringence beat length longerthan 10 m.
 38. The waveguide of claim 35, arranged such thatbirefringence induced by increasing fluid pressure opposes (the)intrinsic birefringence of the waveguide.
 39. The waveguide of claim 35,having a value of intrinsic birefringence selected such that thewaveguide has substantially zero birefringence at a pressure offset fromatmospheric pressure to define a range of fluid pressures detectableusing the waveguide.
 40. The waveguide of claim 35, having deformabilityselected to control the rate of change of induced birefringence withincreasing fluid pressure and thereby to define a/the range of fluidpressures detectable using the waveguide.
 41. The waveguide of claim 35,comprising a side-hole optical fibre.
 42. The waveguide of claim 35,comprising an optical fibre having an elliptical core.
 43. The waveguideof claim 35, having birefringence beat length that remains above aroundtwo times the length of light pulses used to interrogate the waveguideover the range of fluid pressures in which it is intended to use thewaveguide.
 44. A method of detecting pressure distribution in a fluidusing an optical waveguide for submersion in the fluid, the opticalwaveguide being adapted to deform asymmetrically transverse to itslength on exertion of substantially isotropic pressure by the fluid suchthat birefringence of the waveguide varies with the exerted pressure,the method comprising: directing pulses of light into the waveguide;detecting polarisation states of light backscattered from the lightpulses in the waveguide; and deriving an indication of the birefringenceof at least part of the waveguide from the detected polarisation statesto detect pressure distribution in the fluid.
 45. The method of claim44, wherein the optical waveguide is a side-hole optical fibre.
 46. Themethod of claim 45, wherein the side-hole fibre has two holes, one oneither side of the fibre's core, and the angle subtended by the radiusof each hole around the centre of the core is less than around 25°. 47.The method of claim 45, wherein the side-hole fibre has two holes, oneon either side of the fibre's core, and the angle subtended by theradius of each hole around the centre of the core is less than around10°.
 48. The method of claim 44, wherein the optical waveguide is aD-shaped optical fibre.
 49. The method of claim 44, comprising detectingthe polarisation state of light following backscattering at differentpoints along the length of the waveguide and wherein the at least partof the waveguide for which the indication of birefringence is determinedis between at least some of these points.
 50. The method of claim 44,comprising deriving an indication of the variation of birefringencealong the length of the waveguide to detect spatial pressuredistribution in the fluid.
 51. The method of claim 44, comprisingderiving an indication of the birefringence of the at least part of thewaveguide more than once to detect temporal pressure distribution in thefluid.
 52. The method of claim 44, wherein the optical waveguide isplaced in a down-hole of an oil well and the pressure distribution offluid in the down-hole is detected.
 53. The method of claim 44, whereinthe optical waveguide is placed in a water pipe and the pressuredistribution of water in the water pipe can be detected.
 54. A method ofdetecting fluid flow, the method comprising: using the method of claim44, wherein the derived indication detects the distribution along thewaveguide of the pressure exerted by the fluid on the waveguide; anddistinguishing a component of the indication resulting from static fluidpressure from a component of the indication resulting from fluid flow.55-58. (canceled)
 59. A method of locating an interface between twofluids of different density, the method comprising the method of claim44, wherein the optical waveguide extends through the fluids, andlocating a change in the gradient of the detected pressure distributionalong the waveguide to locate the fluid interface.
 60. (canceled)
 61. Amethod of detecting a tsunami, the method comprising the method of claim44, wherein the optical waveguide extends underwater along an expectedpath of a tsunami, the derived indication is of the distribution alongthe waveguide of the pressure exerted by the water on the waveguide, anddetermining when changes in the distribution are characteristic of atsunami travelling through the water along the expected path. 62-67.(canceled)
 68. The method of claim 44, comprising more than onewaveguide.
 69. The method of claim 44, wherein the optical waveguideextends for several tens of kilometres.
 70. The method of claim 44,wherein the birefringence of the optical waveguide remains low over therange of fluid pressures in which it is intended to use the waveguide.71. A method of detecting pressure distribution in a fluid using awaveguide adapted to deform asymmetrically transverse to its length onexertion of substantially isotropic pressure by the fluid such that thebirefringence of the waveguide varies with exerted pressure, but remainslow over the range of fluid pressures in which it is intended to use thewaveguide.
 72. A method of manufacturing an optical waveguide for use indetecting pressure distribution in a fluid, the waveguide being adaptedto deform asymmetrically transverse to its length on exertion ofsubstantially isotropic pressure by the fluid such that birefringence ofthe waveguide varies with the exerted pressure, the method comprisingselecting the birefringence of the waveguide to remain low over therange of fluid pressures in which it is intended to use the waveguide.73. The method of claim 44, wherein the optical waveguide has lowintrinsic birefringence.
 74. The method of claim 44, wherein the opticalwaveguide has intrinsic birefringence beat length longer than around 10m.
 75. The method of claim 44, wherein the waveguide is arranged suchthat birefringence induced by increasing fluid pressure opposes (the)intrinsic birefringence of the waveguide.
 76. The method of claim 44,comprising selecting a value of intrinsic birefringence such that thewaveguide has substantially zero birefringence at a pressure offset fromatmospheric pressure to define a range of fluid pressures detectableusing the waveguide.
 77. The method of claim 44, comprising selectingthe deformability of the waveguide to control the rate of change ofinduced birefringence with increasing fluid pressure and thereby todefine a/the range of fluid pressures detectable using the waveguide.78. The method of claim 44, comprising an optical fibre having anelliptical core.
 79. The method of claim 44, wherein the birefringencebeat length of the optical waveguide remains above around two times thelength of (the) light pulses (used to interrogate the waveguide) overthe range of fluid pressures in which it is intended to use thewaveguide.
 80. Computer program code adapted to carry out the method ofclaim 44 when processed by a processing means. 81-82. (canceled)
 83. Themethod of claim 71, wherein the optical waveguide has low intrinsicbirefringence.
 84. The method of claim 71, wherein the optical waveguidehas intrinsic birefringence beat length longer than around 10 m.
 85. Themethod of claim 71, wherein the waveguide is arranged such thatbirefringence induced by increasing fluid pressure opposes (the)intrinsic birefringence of the waveguide.
 86. The method of claim 71,comprising selecting a value of intrinsic birefringence such that thewaveguide has substantially zero birefringence at a pressure offset fromatmospheric pressure to define a range of fluid pressures detectableusing the waveguide.
 87. The method of claim 71, comprising selectingthe deformability of the waveguide to control the rate of change ofinduced birefringence with increasing fluid pressure and thereby todefine a/the range of fluid pressures detectable using the waveguide.88. The method of claim 71, comprising an optical fibre having anelliptical core.
 89. The method of claim 71, wherein the birefringencebeat length of the optical waveguide remains above around two times thelength of (the) light pulses (used to interrogate the waveguide) overthe range of fluid pressures in which it is intended to use thewaveguide.
 90. The method of claim 72, wherein the optical waveguide haslow intrinsic birefringence.
 91. The method of claim 72, wherein theoptical waveguide has intrinsic birefringence beat length longer thanaround 10 m.
 92. The method of claim 72, wherein the waveguide isarranged such that birefringence induced by increasing fluid pressureopposes (the) intrinsic birefringence of the waveguide.
 93. The methodof claim 72, comprising selecting a value of intrinsic birefringencesuch that the waveguide has substantially zero birefringence at apressure offset from atmospheric pressure to define a range of fluidpressures detectable using the waveguide.
 94. The method of claim 72,comprising selecting the deformability of the waveguide to control therate of change of induced birefringence with increasing fluid pressureand thereby to define a/the range of fluid pressures detectable usingthe waveguide.
 95. The method of claim 72, comprising an optical fibrehaving an elliptical core.
 96. The method of claim 72, wherein thebirefringence beat length of the optical waveguide remains above aroundtwo times the length of (the) light pulses (used to interrogate thewaveguide) over the range of fluid pressures in which it is intended touse the waveguide.